The VLA provides receivers at L-band (18-20 cm), C-band (6 cm), X-band (3.6 cm), U-band (2.0 cm), K-band (1.2 cm) and Q-band (0.7 cm). These are all located at the Cassegrain focus. P-band (90 cm) and 4-band (400 cm) feeds are mounted at prime focus. The availability of each of these bands and details about frequency coverage and noise characteristics can be found in the OSS.
Radio emission coming into the receivers is amplified and mixed with a signal from a local oscillator (LO) to an intermediate frequency (IF) for further amplification and signal treatment. Both right (R) and left (L) circular polarization is measured at each receiver. At the IF stage, i.e., after the initial amplification, the signal is split. The result is two independent IF pairs, each measuring two polarizations, enabling one to measure radio emission over twice the bandwidth as would be possible with one IF pair.2.1 These four IFs are called A, B, C, and D; IFs A and B are right, and C and D are left circularly polarized. The first IF pair consists of IFs A and C, the second IF pair consists of IFs B and D. The signal from each IF passes through a front-end filter which can be set to 50 (default), 25, or 12.5 MHz2.2. The four IFs(A, B, C, and D) have independent, but identical, paths past the Local Oscillator chain and through the waveguide system and correlator (see Figure 2.1). The signals of one telescope are cross-correlated with those of another to create for each IF pair the following products: RR, RL, LR, and LL. In spectral line work the cross terms, RL and LR, which are required to determine linear polarization, are often not calculated. This leaves only the parallel terms, RR and LL, which can be used to derive the total intensity and circular polarization (see the OSS for further details on polarization observations).
